This invention relates to a sub assembly for connecting a swivel of a drilling unit to a drill string. More specifically, the sub assembly includes a telescopic sub having a preloaded spring. The telescopic sub has three discrete positions allowing it to travel in either direction from a normal position when threading in or out of a connection.
Conventional rotary drilling requires the use of a rotary table, a motor for rotating the table, a kelly and kelly bushings. These drilling systems are being replaced by "top drive" drilling systems which rotate the drill string from above using a drilling motor suspended from or incorporated with a swivel.
Such a power swivel can be installed in a standard derrick or mast. Drilling is accomplished by the powered rotation of a suspended string of drill pipe. A cutting tool or bit is at the bottom end of the drill string which, through the rotational energy supplied by the power swivel, cuts through the earth's formations and deepens the well. As the well is drilled, sections or stands of drill pipe must be periodically added to the top end of the drill string. During the drilling of deep wells, such as oil wells, the drill bit may become worn and/or require replacement before drilling of the well is completed. In this situation the entire drill string must be withdrawn from the well, one stand of pipe at a time, and the drill bit replaced. After a new drill bit is replaced at the bottom end of the stand of drill pipe, the drill string is reassembled as described above. For drilling equipment utilizing a power swivel, the pipe handling operation is remotely controlled from a console on the derrick platform. Because the drilling unit is large and somewhat cumbersome to handle, the threads on the pipe ends frequently become damaged when "spinning in" or "spinning out" the individual pipe stands from the drill string. When the pipe threads become damaged, the pipe must be temporarily taken out of service until the threads can be remachined to serviceable condition. Pipe having damaged threads and not taken out of service for repair could result in a separation of the drill string inside the well bore. This increases the costs of drilling because of delay time and the increased inventory of drill pipe required.
When a drill bit must be replaced, the power swivel vertically lifts the drill string from the well a distance above the floor of the drilling rig corresponding to the length of a stand of pipe. The stand of drill pipe may include as many as three sections of pipe. The weight of the drill string is then supported by wedges or slips located at the floor of the drilling rig. The tongs of pipe handling equipment are used to hold and prevent rotation of the drill string while simultaneously rotating or "spinning out" the top stand of pipe. This top stand of pipe is supported by the power swivel as the power swivel rotates the stand of pipe. As the stand of pipe becomes threadably disengaged from the drill string, the tension of the power swivel causes the stand of pipe to be abruptly pulled away. However, the bottom end of the stand of pipe frequently is impacted against the top end of the drill string caused by the rebound of the abrupt disengagement. Similar rebound and impact may occur when disconnecting the sub assembly of the power swivel from the top end of the pipe. Thread damage may occur when reassembling the stands of pipe to the drill string. The operator's visibility may be partially obscured when lowering a stand of pipe using the power swivel and the control means lacks sensitivity to prevent sudden impact when engaging the bottom end of the stand of pipe to the top end of the drill string during the "spinning in" operation. For a stand of pipe extending as much as 90 feet (27.4 m) above the console, the operator's line of sight may be completely obscured when connecting the sub assembly of the power swivel to the upper end of the stand of pipe.
There have been many attempts over the years to provide cushioning devices to prevent impact and thread damage when "spinning out" or "spinning in" pipe to a drill string. However, these devices do not provide cushioning or thread protection when connecting and disconnecting a swivel sub assembly to a drill string. More importantly, these cushioning devices have not completely eliminated impacts because thread damage still occurs. U.S. Pat. No. 2,712,932 discloses a telescoping cushioning device including a helical compression spring positioned within a cylinder. The cylinder is positioned at the top end of a pipe to be connected to a drill string. The spring yields or is compressed by the weight of the pipe so that the lower end of the pipe can be connected to the drill string without danger of thread damage. U.S. Pat. No. 3,766,991 discloses a power swivel utilizing shock absorbers enabling smooth handling of pipe. The hangers for the swivel are formed by hydraulic cylinders so that upward movement of the power swivel resulting when the pipe section is threaded is compensated for by the downward movement of a piston under the cushioning pressure of a fluid. This assures smooth release of the threads without sudden upward movement of the power swivel when the threads and released. U.S. Pat. No. 4,593,773 discloses a drilling assembly including a top driven rotary device. The rotary device is connected to a drill string using a telescopic shaft. When a pipe is hoisted by the rotary device, hydraulic fluid is exhausted from external hydraulic cylinders allowing pipe supporting hangers to be pulled downwardly.
Nevertheless, the above described cushioning or telescopic devices do not eliminate the impact problem and provide the same degree of thread protection during both pipe handling situations discussed above. There remains a long-felt need for a device which eliminates impact and thread damage both when making and breaking connections to drill pipe and a sub assembly. My invention overcomes this problem by providing a sub assembly including a spring loaded telescopic sub having three discrete positions. The sub can travel in either direction from its normal position. When a swivel is lowered by a support means for connection to a drill pipe, the sub will travel upwardly relative to the swivel when the sub is lowered into contact with the drill pipe. When the swivel is connected to the drill pipe, the make-up of the threads causes the sub to travel downwardly relative to the swivel thereby compressing the spring. When the sub is disconnected from the drill pipe, expansion of the spring causes the sub to be pulled away from the drill pipe when the threads are disengaged as the sub moves upwardly relative to the drill pipe back to its normal position.